1. Field of the Invention
This invention relates to an oxygen concentration-detecting device for internal combustion engines, which detects the concentration of oxygen present in exhaust gases emitted from an internal combustion engine, and more particularly to an oxygen concentration-detecting device of this kind which employs an oxygen sensor of a limit current type (hereinafter referred to as "the LAF sensor").
2. Prior Art
Conventionally, an oxygen concentration-detecting device of this kind has been proposed, e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 59-163556, which utilizes an output from the LAF sensor having a characteristic that when the temperature of the LAF sensor is within an activation temperature range, the sensor output current exhibits a value proportional to partial pressure of oxygen in exhaust gases if a certain positive voltage is applied to the sensor, to thereby linearly detect the concentration of oxygen present in the exhaust gases. The LAF sensor has a limit current characteristic which varies with the temperature thereof. Therefore, to maintain the accuracy of detection of the oxygen concentration, it is required to constantly control the temperature of the LAF sensor such that it is within the activation temperature range. To meet this requirement, the proposed oxygen concentration-detecting device utilizes a characteristic of the LAF sensor that when a negative voltage is applied to the sensor, the value of the sensor output current is not dependent on the partial pressure of oxygen but proportional to the temperature of the LAF sensor. More specifically, the proposed device detects the internal resistance of the LAF sensor by applying a predetermined negative voltage to the sensor, and heats the LAF sensor with a heater such that the detected internal resistance of the LAF sensor is held constant, to thereby maintain the LAF sensor in the active state.
However, since the oxygen concentration in exhaust gases and the internal resistance of the LAF sensor are detected by applying respective different values of voltage (the predetermined positive voltage and the predetermined negative voltage) to the LAF sensor, they cannot be detected concurrently. Therefore, the applied voltage is changed over between the predetermined positive voltage and the predetermined negative voltage at predetermined time intervals (in a time-sharing manner) to thereby alternately detect them.
Generally, the output from the LAF sensor is utilized for various kinds of control. For example, the output from the LAF sensor is used in air-fuel ratio feedback control of an internal combustion engine based on a modern control theory, in which fine control is carried out based on the output from the LAF sensor by using a controller of a recurrence formula type, such as an observer and an optimal regulator. In this control, the optimal value has to be selected from a group of samples of the LAF sensor output. This requires the sensor output to be sampled at a very short sampling period.
In the oxygen concentration-detecting device proposed by Japanese Laid-Open Patent Publication (Kokai) No. 59-163556, however, the detection of the oxygen concentration is suspended at least during a time period over which the internal resistance of the LAF sensor is detected, and therefore actual or true detected values of the oxygen concentration are not obtained during the time period. Moreover, the time period of detection of the internal resistance of the LAF sensor comes round at the predetermined time intervals irrespective of operating conditions of the engine (hereinafter, the period at which comes round the time of detection of the internal resistance will be referred to as "the internal resistance detection repetition period"). Therefore, if the internal resistance detection repetition period is set to a very short period, the detection of the oxygen concentration is suspended more frequently, which causes the observer, for example, to make inaccurate estimation of air-fuel ratios of exhaust gases from the cylinders of the engine (cylinder-by-cylinder air-fuel ratio), resulting in inability to properly achieve fine control as intended.
More specifically, in the observer, a value of the oxygen concentration detected immediately before the start of detection of the internal resistance of the LAF sensor is held until the start of the next detection of the oxygen concentration, and the held value of the oxygen concentration is regarded-as values of the same detected over the time period during which the detection of the oxygen concentration is suspended (suspension period). If the held value is selected as samples of the LAF sensor output to be obtained during the suspension period, the value of the oxygen concentration thus selected as a value for the desired cylinder (e.g. #2) can be closer to the actual value of the oxygen concentration of exhaust gases from the immediately preceding cylinder (e.g. #4) than a value which should be actually detected from exhaust gases from the desired cylinder (#2). In such a case, the difference between the air-fuel ratio value estimated based on the above held value and the actual or true air-fuel ratio value becomes large, leading to inaccurate estimation of the cylinder-by-cylinder air-fuel ratio. As a result, the air-fuel ratio feedback control cannot converge the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio or the air-fuel ratio of the mixture diverges. Especially, when the operating cycle of the engine and the internal resistance detection repetition period are almost synchronous with each other, it can occur that the air-fuel ratio of exhaust gases from a particular cylinder cannot be accurately detected at all, exerting a large adverse effect on the estimation of the cylinder-by-cylinder fuel-air ratio.
Thus, the proposed oxygen concentration-detecting device still remains to be improved in reaching the full potentials of various kinds of fine control which use values of the oxygen concentration detected by the LAF sensor, such as control based on the modern control theory.
Further, even in a type of control which does not require values of the LAF sensor output sampled at a short sampling period, if the frequency of coincidence of timing requiring a detected value of the oxygen concentration with the internal resistance-detecting period is high, this can degrade the air-fuel ratio controllability. Therefore, to decrease the frequency of coincidence, it is desirable to set the internal resistance detection repetition period as long as possible.
However, if the internal resistance detection repetition period is set too long, it is difficult to constantly maintain the temperature of the LAF sensor within the activation temperature range. For example, a delay of control of the temperature of the sensor element of the LAF sensor occurs due to the excessively long internal resistance detection repetition period when the temperature of exhaust gases suddenly changes, e.g. upper fuel cut of the engine. As a result, the accuracy of detection of the oxygen concentration by the LAF sensor is degraded.
Thus, if the internal resistance detection repetition period is uniformly set to a fixed value, there can arise inconveniences depending on operating conditions of engine.